Concept:The instrument used for high-frequency alternating current must work on a principle that does not depend on frequency.
Explanation:The hot wire ammeter works on the heating effect of current. When current passes through a thin wire, the wire heats up and expands, causing the pointer to move.
The heat produced is proportional to
I2, and this effect is the same for alternating current and direct current of equal effective value.
Because heating does not change with frequency, the hot wire ammeter gives accurate readings even at high frequencies.
A moving coil ammeter measures the average current, which is zero for pure AC.
A moving iron ammeter depends on magnetic effects that change with frequency, giving errors at high frequencies.
Therefore, the hot wire ammeter is most suitable for measuring high-frequency a.c.
Answer:D. hot wire ammeter.